The present invention relates to a differential refractometer to be used for determining a molecular weight for example.
There is known a technique for measuring the refractive index of a solution and the dependency thereof on the concentration to measure the molecular weight of a sample which is a solute of the solution. The refractive index of a sample solution may be measured, for example, with the use of a cell having a transparent container in which the sample solution and its solvent are housed as separated from each other. When monochromatic light is incident upon the cell through a slit, the optical path of the incident light is deflected according to the difference in refractive index between the sample solution and the solvent. Accordingly, when the refractive index of the solvent is known, the refractive index of the sample solution may be obtained. Thus, it is a differential refractometer that is adapted to measure the refractive index of a sample with the use of the fact that the incident light is deflected according to the difference in refractive index between a reference of which refractive index is known (the solvent of the sample solution in the example above-mentioned) and a sample of which refractive index is unknown (the sample solution).
The basic arrangement of a conventional differential refractometer is shown in FIG. 21. Light from a light source 1 is guided, through a slit plate 2, to a collimater lens 3 where the light is collimated. The parallel light portions are incident upon a cell 5, as a light flux A of which width is restricted by a slit plate 4. The light flux A from the cell 5 passes through an image forming lens 6 and a correction glass plate 7 to be discussed later, and forms a slit image formed by the slit plate 4 on a detecting surface of a photosensor 8 disposed at the side of the focal surface of the image forming lens 6. The correction glass plate 7 may be angularly displaced in a direction shown by an arrow R1 by operating a dial 9.
FIG. 22 is an enlarged transverse section view of the cell 5 showing the arrangement thereof. This cell 5 is called a Brice cell or the like in which a cell container 5a is formed by a transparent casing body having a rectangular section and in which the inside space of the cell container 5a is obliquely partitioned by a partitioning plate 5b to form a first chamber 51 and a second chamber 52. For example, when the first chamber 51 is filled with a sample solution of which refractive index is to be measured, and the second chamber 52 is filled with a solvent for the sample solution, the light flux A from the slit plate 4 is deflected according to the difference in refractive index between the sample solution and the solvent.
As the first step for measuring the refractive index, the first chamber 51 and the second chamber 52 are filled with the same solvent, and the slit image formed by the slit plate 4 is detected by the photosensor 8. As the second step, for example the first chamber 51 is filled with the sample solution and the second chamber 52 is filled with the solvent and the similar measurement is carried out. The positions of the slit images formed at the first and second steps, are different from each other correspondingly to the amount of deflection of the light flux A which is produced according to the difference in refractive index between the sample solution and the solvent.
The difference in refractive index .DELTA.n is expressed according to the following equation (1): ##EQU1## where n.sub.S :Refractive index of the sample solution
n.sub.R :Refractive index of the solvent PA1 e:Refractive index outside of the cell PA1 .alpha.:Deflection angle of the light flux A PA1 .theta.:Angle formed between the light flux A and the partitioning plate 5b
In the equation (1), either positive or negative sign is selected according to the refractive indexes n.sub.S, n.sub.R and the angle .theta.. On the other hand, when the distance between the cell 5 and the photosensor 8 is defined as l and the variation of slit image forming position .DELTA.X is used, sin .alpha. is expressed according to the following equation (2): ##EQU2##
Accordingly, the equation (1) is transformed to the following equation (3): ##EQU3##
Since e is approximately equal to 1 in the air, the difference in refractive index .DELTA.n is finally expressed by the following equation (4): ##EQU4##
More specifically, when the displacement .DELTA.X is known, the difference in refractive index .DELTA.n may be obtained. Accordingly, the refractive index of the sample solution may be obtained based on the refractive index of the solvent.
FIG. 23 is a plan view illustrating the operation of the correction glass plate 7. When the dial 9 is operated to angularly displace the correction glass plate 7 by an angle .beta. from a reference position 7a (shown by a broken line in FIG. 23), the incident angle of the light flux A upon the correction glass plate 7 is equal to the angle .beta.. In this state, when it is presumed that the light flux A is refracted so that the optical path thereof is shifted by .DELTA.xa, such a displacement of the optical path .DELTA.xa is approximately proportional to sin .beta. as follows: EQU xa .varies. sin .beta......... (5)
To measure the difference in refractive index .DELTA.n, with the correction glass plate 7 assuming a posture of the reference position 7a, the both chambers 51, 52 of the cell 5 are first filled with the solvent and the light flux A is detected by the photosensor 8. Then, the first chamber 51 of the cell 5 is filled with the sample solution and the dial 9 is operated such that the deflected light flux A is detected by the photosensor 8. In this state, the displacement .DELTA.xa of the optical path of the light flux A by the correction glass plate 7 is expressed by the following formula (6): EQU .DELTA.xa.varies..DELTA.x..... (6)
The displacement .DELTA.xa may be obtained from the value of the dial 9. Accordingly, based on the value of the dial 9, the difference in refractive index between the sample solution and the solvent .DELTA.n may be obtained with the use of the equation (4).
Such an arrangement, however, involves the likelihood that there is a individual difference among the operators when the dial 9 is manually operated to shift the slit image forming position. This lowers the reproducibility of data, resulting in deterioration of precision in measurement of refractive index.
Further, there is a certain time interval between the detection of a slit image formed at the time when both the chambers of the cell 5 is filled with the solvent and the detection of a slit image formed at the time when the first chamber 51 of the cell 5 is filled with the sample solution. This involves error factors such as variations of the mechanical vibration and air fluctuation with the passage of time, the deflection of an optical base (not shown) with the passage of time, and the like. This further deteriorates the measuring precision.
Other prior art is disclosed by, for example, JP-A-188744/1988 of which basic arrangement is shown in FIG. 24. Light from a light source 11 is condensed at a condensing lens 12, spatially filtered by a slit plate 13 and collimated by a collimater lens 14. The resultant parallel light portions are incident upon a V-block 15 made of a transparent material of which refractive index is known. The V-block 15 has a V-shape concave 15a having a vertical angle of 90.degree. . The concave 15a serves as a sample stand. Placed on the concave 15a is a sample 16 which has a vertical angle of, for example, 90.degree. and of which refractive index is unknown. Almost a half of the parallel light portions from the collimater lens 14 passes through the sample 16.
The light portions from the V-block 15 pass through a chopper 17 and are condensed by an image forming lens 18. The slit image formed by a slit plate 13 is then formed on the light receiving surface of a one-dimensional image sensor 19 formed by a one-dimensional CCD (charge coupled device) or the like. The chopper 17 has a stationary piece 17a and a movable piece 17b. The stationary piece 17a includes an opening for receiving the light portion which has passed through the sample 16 and an opening for receiving the light portion which has not passed through the sample 16. The movable piece 17b closes either one of these two openings.
To measure the refractive index of the sample 16, the light portion which has passed through the sample 16 is first intercepted by the chopper 17. The slit image forming position in this state is detected by the one-dimensional image sensor 19 and held by, for example, control means (not shown). In this case, the light portion passing through the chopper 17 is the light portion which has passed through only the V-block 15 of which refractive index is uniform, so that this light portion is not deflected.
Then, the chopper 17 intercepts, out of the light portions from the V-block 15, the light portion which has not passed through the sample 16. Thus, incident upon the image forming lens 18 is the light portion which has been deflected correspondingly to the difference in refractive index between the V-block 15 and the sample 16. This causes the slit image to be formed on a position different from the position above-mentioned, such a positional difference corresponding to the difference in refractive index. This image position is detected by the one-dimensional image sensor 19 and supplied to the control means above-mentioned. Calculated in the control means is the distance between two positions at which the slit images are respectively formed by the light portion having passed through the sample 16 and the light portion not having passed therethrough. From the result of this calculation, the difference in refractive index between the sample 16 and the V-block 15 is obtained in the same manner as in the first prior art shown in FIG. 21.
In the prior art shown in FIG. 24, the amount of light deflection is measured based on an output from the one-dimensional image sensor 19 with no manual operation required. This not only facilitates the measurement of refractive index, but also contains no measuring errors due to the individual difference among the operators, thereby to improve the measuring precision.
However, when the movable piece 17b of the chopper 17 is driven, vibration is produced to displace the slit image forming position on the one-dimensional image sensor 19. This results in deterioration of the measuring precision. Further, the incorporation of the mechanically driven member inevitably increases the number of component elements.